Diarra made a big impact at Craven Cottage when he joined the Premier League side early in 2012, but frequent injury problems have resulted in Jol admitting that he is unsure whether the Mali international will remain at the club.
The 57-year-old told reporters: "I think it will be quite complicated without an operation to get him fit again.
"He was the main man last season when he came in. It was quite a boost to get a player of his quality but you have to be fit. If not, he can do nothing for us.
"Hopefully in a few months we will sit down, have a look at him and make a decision."
The 31-year-old moved to Fulham after being released by Monaco.